RDS Extended Support for MySQL

Hello.

If you look at the document below, it states that the additional charges will automatically stop if you upgrade or delete the target RDS.
So, if you have upgraded, I think there will be no problem.
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/extended-support.html

The additional charge for RDS Extended Support automatically stops as soon as you delete the database that is running a major version past the RDS end of standard support date, or you upgrade to an engine version that is covered under standard support. However, the charges will restart if your target engine version enters Extended Support in the future.

If you are still being charged, I recommend that you open a case with AWS Support under "Account and billing".
Inquiries under "Account and billing" can be made free of charge.
https://docs.aws.amazon.com/awssupport/latest/user/case-management.html